1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to an optical setup for writing high-quality interference patterns with a well characterized period and chirp.
2. Discussion of the Related Art
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. Commonly, amplitude modulation of the optical carrier is used. However, this approach suffers from poor signal to noise ratio. It is well known that broadband modulation schemes that utilize higher bandwidth than that of the transmitted waveform may improve the signal to noise ratio over that using amplitude modulation. Pulse position modulation (PPM) is one such technique. In pulse position modulation, a shift in the pulse position represents a sample of the transmitted waveform. This is shown in FIG. 1. It can be shown that for a given power source SNRPPM∝SNRAM (tP/τ)2 where tp is the spacing between the unmodulated pulses and τ is the pulse duration, respectively. Pulse position modulation for optical communications requires new techniques for generating trains of optical pulses whose positions are shifted from their unmodulated positions in proportion to the amplitude of a transmitted waveform. Various types of devices are known to those skilled in the art. Such devices utilize distributed feedback reflector in electro-optically active waveguides. Techniques for forming distributed feedback methods are inconsistent.
One of the steps in creating a distributed feedback reflector includes creating an interference pattern on a layer of photo resist. The photo resist is then exposed, for example, to ultra-violet light in order to create a desired pattern. The interference patterns for surface and volume holograms are generally created with one of the following: wavefront-splitting interferometers, phase-masks, and amplitude-splitting interferometers.
Wavefront-splitting interferometers provide two interfering beams that are carved from different areas of the wavefront of a spatially coherent beam. Such splitting however, results in diffraction at the boundary of the cut, causing parasitic fringes.
Phase-masks are illuminated by a single laser beam, creating interfering beams on a closely positioned target. Because of its simplicity, this technique has been very popular for writing fiber Bragg gratings with uniform periods. Phase masks may also be used for making linearly-chirped gratings. The major drawback of this method is due to the imperfections of available phase masks. Usually, a third beam (zero-order diffraction) is present at the output of the phase mask in addition to the desirable first-order diffracted beams. Since the intensity of the parasitic beam is relatively high (typically 3–5%), it causes strong (more than 50%) parasitic modulation of the hologram at a double period.
Amplitude splitting interferometers create two interfering beams by splitting a parent beam in two on a partially-reflecting beam splitter. The beams are then spatially shaped and combined on a target. With this technique, large-area holograms have been written using beam expanders. Furthermore, interference patters with linear and quadratic chirps have been manufactured employing optical beams with spherical wavefronts. This technique, however, requires considerable beam expansion for achieving uniform illumination across large areas. As has been already mentioned, this creates unwanted diffraction on system apertures.
It would therefore be desirable to provide a system for writing high-fidelity gratings with well-characterized periods and chirps.